1. Field of the Invention
The present invention relates to a method and system for controlling a wind farm when a ramp up or ramp down rate of the wind farm deviates from a grid-code or corresponding criteria due to an abrupt change in wind speed, and more specifically, to a method and system for determining a sequence of controlling wind turbines in the wind farm using information on wind conditions measured outside the wind farm and for controlling the wind turbines according to the control sequence so that power output of the wind farm may satisfy the ramp up or ramp down rate specified in the grid-code or corresponding criteria.
2. Background of the Related Art
A wind turbine is an apparatus for converting kinetic energy of wind into electrical energy. Speed of wind should be within a certain range in order to generate wind power. If the wind speed exceeds the upper limit of wind power generation reference speed (e.g., 25 m/s), the wind turbine should be stopped to protect the wind turbine, and if the wind speed is less than the lower limit (e.g., 3 m/s), the wind turbine stops working since it does not have sufficient energy as much as to generate power.
When the wind turbine is to be stopped, input energy is reduced to zero by controlling a pitch angle through a pitch control, and a brake is used when the wind turbine should be stopped in a speedy way. However, if the brake is used, it is disadvantageous in that the wind turbine is abraded and thus the lifespan of the wind turbine could be shortened.
Meanwhile, since the electrical energy generated by the wind turbine is affected by the strength of fluctuating wind and thus cannot maintain a constant level at all times, its quality is inferior to those of conventional power generators. In order to maintain electrical energy of high quality, a power generator in a power grid should reserve sufficient power so as to compensate increase and decrease in power output of the wind turbine. However, a large amount of power should be reserved since the power output of the wind turbine is highly variable, and thus cost of generating the electrical energy increases as a result. Since this problem is worsened if a large number of wind turbines are associated with the power grid, a grid-code is established and enforced in a lot of countries of the world, and the grid-code is also announced in Korea in June, 2010.
Since power generators in a conventional power grid have a ramp up or ramp down rate of a definite value, the grid-code specifies a regulation for increasing and decreasing power output of a wind farm while maintaining a ramp up or ramp down rate of the power output of the wind farm to be smaller than a certain value in any circumstances. If the power output increases or decreases at a rate higher than specified in the regulation, the power generators in the conventional power grid cannot compensate abrupt increase and decrease of the power output of the wind farm, and thus quality of the electrical energy is degraded.
In addition, when a strong wind blows, the wind turbine should be forcibly stopped as described above in order to protect the wind turbine. However, in the case of a large-scaled wind farm, power output of the wind farm abruptly decreases if all wind turbines simultaneously stop working, and this greatly affects power output of the power grid. On the other hand, if there is no wind, power output of the wind farm abruptly decreases since the wind turbines stop working, and it gives a great affect to the power grid, and thus the wind turbines should be forcibly stopped in order to reduce the affect.
In order to forcibly stop the wind turbines in the wind farm when a strong wind blows, a conventional technique calculates a distance sij to each wind turbine and a time required for the wind to travel from a measuring apparatus to the wind turbine sij/v. In this method, it is assumed that a time required to forcibly stop an individual wind turbine is tdown. If the time consumed for calculation and communication is ignored, each wind turbine is forcibly stopped at a time of tij(sij/v−tdown) for a time period of tdown.
Since the wind turbines are forcibly stopped for a time period of tdown, the number of simultaneously stopped wind turbines varies, and thus power output of the wind farm forms a curve. In addition, the ramp down rate is low at a moment when the number of stopped wind turbines is small, whereas the ramp down rate is high when the number of stopped wind turbines is large, and thus the ramp down rate may exceed the grid-code.